{"gene":"CBLN1","run_date":"2026-06-09T22:57:17","timeline":{"discoveries":[{"year":2010,"finding":"Cbln1 forms a trans-synaptic tripartite complex bridging presynaptic neurexins (NRXNs) on granule cells to the N-terminal domain (NTD) of postsynaptic GluRδ2 (GluD2) on Purkinje cells, mediating cerebellar synapse formation. Synaptogenic activity of GluD2 is abolished in cbln1-null cultures and restored by recombinant Cbln1; knockdown of NRXNs in granule cells also abolishes the synaptogenic activity. Soluble NTD of GluD2 and extracellular domain of NRXN1β each suppress Cbln1 synaptogenic activity in vitro and in vivo.","method":"Cerebellar primary cultures from cbln1-null mice, recombinant Cbln1 rescue, NRXN knockdown, competitive inhibition with soluble domain fragments, in vivo injection","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (genetic KO, recombinant rescue, domain competition, in vivo), independently replicated across labs","pmids":["20537373"],"is_preprint":false},{"year":2010,"finding":"Cbln1 directly binds the N-terminal domain of GluD2 (the orphan glutamate receptor). The Cbln1–GluD2 complex acts as a bidirectional synapse organizer: Cbln1-coated beads directly induce presynaptic differentiation and indirectly cause postsynaptic molecule clustering via GluD2. GluD2 postsynaptic expression combined with exogenous Cbln1 is necessary and sufficient to induce new synapses in vitro and in adult cerebellum in vivo.","method":"Direct binding assays, bead-coated recombinant Cbln1 synaptogenesis assay, in vitro and in vivo synapse induction experiments","journal":"Science","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — direct binding shown, bead reconstitution, in vitro and in vivo synaptogenesis, replicated by multiple labs","pmids":["20395510"],"is_preprint":false},{"year":2005,"finding":"Cbln1 is a glycoprotein secreted from cerebellar granule cells that is essential for (1) maintaining pre- and postsynaptic elements at parallel fiber–Purkinje cell synapses, (2) establishing proper climbing fiber–Purkinje cell innervation, and (3) induction of long-term depression at parallel fiber–Purkinje cell synapses. cbln1-null mice phenocopy GluRδ2-null mice, establishing Cbln1 in the same signaling pathway as GluRδ2.","method":"cbln1 knockout mice, genetic epistasis (phenocopy of GluRδ2-null), electrophysiology (LTD), morphological analysis","journal":"Nature Neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean KO with multiple defined phenotypic readouts, genetic epistasis, replicated across multiple subsequent studies","pmids":["16234806"],"is_preprint":false},{"year":2000,"finding":"Cbln1 forms homomeric hexameric complexes and also interacts specifically with Cbln3 to form heteromeric complexes, mediated by conserved N-terminal cysteine residues (for higher-order oligomerization) and the C-terminal C1q domain (for trimerization). C1qB binds to neither Cbln1 nor Cbln3, demonstrating specificity. Cbln3 cannot form stable homomers.","method":"Yeast two-hybrid screen using Cbln1 as bait, mammalian co-expression, binding specificity assays","journal":"The Journal of Neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — yeast two-hybrid plus mammalian expression, two orthogonal methods, single lab","pmids":["10964938"],"is_preprint":false},{"year":2005,"finding":"Cbln1 undergoes proteolytic processing in the synaptic compartment. Only uncleaved Cbln1 (containing the cerebellin motif) is released and assembles into hexameric complexes. Cleavage at the N-terminus of the cerebellin sequence yields trimeric complexes by separating the C-terminal C1q domain from N-terminal cysteine residues. Cleavage at the C-terminus of the cerebellin motif disrupts the C1q domain and abolishes subunit interactions.","method":"Biochemical analysis of cerebellar lysates, yeast two-hybrid, mammalian expression systems, protease cleavage mapping","journal":"Journal of Neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro cleavage assays plus yeast two-hybrid and mammalian expression, single lab, two orthogonal methods","pmids":["16135095"],"is_preprint":false},{"year":2006,"finding":"Cbln1 is secreted from cerebellar granule cells in complex with Cbln3. Cbln1 and Cbln3 reciprocally regulate each other's degradation and secretion: cbln1-null mice lack both Cbln1 and Cbln3, whereas cbln3-null mice lack Cbln3 but have ~6-fold increased Cbln1. Cbln3 cannot form homomers and is secreted only when bound to Cbln1. A single arginine residue in Cbln3 causes ER retention via steric clash that is masked upon Cbln1 binding ('hide-and-run' mechanism).","method":"cbln1 and cbln3 knockout mice and double knockouts, structural modeling, mutation analysis, biochemical secretion assays","journal":"Molecular and Cellular Biology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — genetic KOs with defined phenotypes, structure-based mutation analysis, biochemical secretion assays, multiple orthogonal methods","pmids":["17030622"],"is_preprint":false},{"year":2008,"finding":"Recombinant Cbln1 specifically and reversibly induces parallel fiber (PF) synapse formation in dissociated cbln1-null Purkinje cells in culture and rapidly induces functional, ultrastructurally normal PF synapses in acute cbln1-null cerebellar slices. A single injection of recombinant Cbln1 in adult cbln1-null mice in vivo completely but transiently restores PF synapses and rescues ataxia.","method":"Dissociated cbln1-null Purkinje cell cultures, acute slice preparations, in vivo injection of recombinant Cbln1, electrophysiology, electron microscopy","journal":"The Journal of Neuroscience","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — in vitro reconstitution, ex vivo, and in vivo rescue, multiple orthogonal assays (electrophysiology + ultrastructure + behavior)","pmids":["18524896"],"is_preprint":false},{"year":2007,"finding":"Cbln1 is localized to the endolysosomal compartment of neurons (co-localizing with cathepsin D, a lysosomal marker) but not to ER or Golgi. In cbln3-null cerebellum, Cbln1 immunoreactivity increases dramatically, while it is unchanged in extracerebellar neurons, indicating that Cbln3 regulates Cbln1 levels specifically in the cerebellum via the endolysosomal pathway.","method":"Immunohistochemistry with organelle markers, cbln1-lacZ transgenic mice, cbln3-null mice, subcellular fractionation","journal":"European Journal of Neuroscience","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — immunohistochemistry co-localization with validated markers, confirmed using transgenic and KO models, single lab","pmids":["18001291"],"is_preprint":false},{"year":2009,"finding":"Cbln1 accumulates specifically in the synaptic cleft of parallel fiber–Purkinje cell synapses (not other PC synapses), co-localizing with Cbln3 and GluRδ2 at these synapses. Intact hexameric Cbln1 specifically binds to postsynaptic sites; neither N-terminal nor C-terminal fragments alone, nor trimeric mutant Cbln1, support specific binding. Cbln1 binding site is located on postsynaptic (Purkinje cell) rather than presynaptic elements.","method":"Postembedding immunogold electron microscopy, pepsin antigen retrieval, binding assays with recombinant Cbln1 in weaver and pcd mutant cerebellum, subcellular fractionation","journal":"European Journal of Neuroscience","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (EM immunogold, binding assays with domain mutants, mutant mouse cerebellum, subcellular fractionation), rigorous controls","pmids":["19250438","19200061"],"is_preprint":false},{"year":2009,"finding":"Cbln1 undergoes anterograde trans-neuronal transport from granule cells to Purkinje cells and Bergmann glia, entering the endolysosomal trafficking system. Cbln1 is absent in Purkinje cells and Bergmann glia of GluRδ2-null mice, suggesting GluRδ2 is required for Cbln1 trafficking into postsynaptic cells. Ectopic Cbln1 expression in Purkinje cells (L7-cbln1 transgene) partially rescues locomotor deficits of cbln1-null mice.","method":"Immunohistochemistry in transgenic and GluRδ2-null mice, L7-cbln1 transgenic mice, behavioral analysis","journal":"Molecular and Cellular Neurosciences","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — genetic models with defined cellular localization outcomes, single lab, multiple genetic backgrounds tested","pmids":["19344768"],"is_preprint":false},{"year":2009,"finding":"Neuronal activity (elevated K+ or kainate) decreases cbln1 mRNA expression in mature granule cells within hours, in a manner dependent on L-type voltage-dependent Ca2+ channels and calcineurin. Chronic activity also reduces Cbln1 protein levels, accompanied by reduction of excitatory synapses on Purkinje cell dendrites; this activity-induced synapse reduction is prevented by exogenous Cbln1.","method":"Granule cell cultures with pharmacological manipulation (K+, kainate, L-type Ca2+ channel blockers, calcineurin inhibitors), qRT-PCR, immunohistochemistry, synapse counting","journal":"The Journal of Neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacological dissection of signaling pathway, rescue experiment with recombinant Cbln1, two orthogonal methods, single lab","pmids":["19403810"],"is_preprint":false},{"year":2012,"finding":"Cbln1 released from parallel fibers induces dynamic structural changes (protrusions forming circular structures that encapsulate Purkinje cell spines) in presynaptic parallel fibers through a mechanism requiring postsynaptic GluD2 and presynaptic neurexin (Nrx). Nrx–Cbln1–GluD2 signaling induces accumulation of synaptic vesicles and GluD2, leading to mature synapse formation via a positive feedback mechanism.","method":"Time-lapse imaging in organotypic culture, ultrastructural analysis in vivo, genetic manipulation (GluD2/Nrx ablation)","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — live imaging plus ultrastructural analysis plus genetic epistasis (multiple orthogonal methods), single lab","pmids":["23141067"],"is_preprint":false},{"year":2012,"finding":"Cbln1 and Cbln2 have similar binding activities to β-neurexins and Grid2 (GluD2). Ectopic Cbln2 expression in Purkinje cells of transgenic mice rescues cerebellar deficits in cbln1-null mice, demonstrating functional redundancy mediated by common receptor binding properties. However, Cbln2 does not substitute for Cbln1 in thalamic neurons affecting striatal synapses, implying region-specific receptor/mechanism differences.","method":"Binding assays, Cbln2 transgenic rescue of cbln1-null mice, cbln2 knockout mice, synaptic analysis","journal":"Journal of Neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — transgenic rescue and binding assays, two orthogonal approaches, single lab","pmids":["22117778"],"is_preprint":false},{"year":2014,"finding":"Cbln1–GluD2 signaling downregulates the formation and function of inhibitory synapses between Purkinje cells and interneurons. cbln1-null cerebellum shows increased density of interneuron–Purkinje cell inhibitory synapses and increased amplitude and frequency of miniature IPSCs. Recombinant Cbln1 reverses increased inhibitory currents and synapse density; this effect is absent in cbln1/GluD2 double-null mice. Tyrosine phosphorylation is upregulated in cbln1-null cerebellum, and Src-family kinase inhibition suppresses increased IPSCs.","method":"Immunohistochemistry (vGAT antibody), whole-cell patch-clamp in cerebellar slices, recombinant Cbln1 rescue, double-knockout epistasis, Src kinase inhibitors","journal":"European Journal of Neuroscience","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (electrophysiology, morphology, pharmacology, genetic epistasis), rigorous controls including double KO","pmids":["24467251"],"is_preprint":false},{"year":2017,"finding":"Crystal structures of the homotrimeric C1q domains of Cbln1 and Cbln4 resolved at 2.2 Å and 2.3 Å, respectively, reveal that divergence in loop CD accounts for the difference in GluD2 binding between Cbln1 and Cbln4. Negative-stain EM reconstruction of hexameric full-length Cbln1 at 13 Å and Cbln4/Nrxn1β complex at 19 Å shows that Nrxn1β binds to the N-terminal region of Cbln4 (through strand β10 of S4 insert). Cbln4 binds Nrxn1β and forms a stable complex with the LNS domain of Nrxn1β despite not binding GluD2.","method":"X-ray crystallography, negative-stain electron microscopy, binding assays","journal":"Cell Reports","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structures plus EM reconstruction plus binding assays, single lab, multiple orthogonal methods","pmids":["28877468"],"is_preprint":false},{"year":2019,"finding":"Cbln1 is released from lysosomes in axons (but not dendrites) of cerebellar granule cells in an activity- and Ca2+-dependent manner. Released Cbln1 is retained on axonal surfaces by binding to presynaptic neurexin, then diffuses laterally and accumulates at boutons by binding postsynaptic δ2 glutamate receptors. Cbln1 exocytosis is insensitive to tetanus neurotoxin, is accompanied by cathepsin B release, and is decreased by lysosome disruption. Overexpression of lysosomal sialidase Neu1 inhibits Cbln1 and cathepsin B exocytosis and reduces axonal bouton formation in vivo.","method":"Live-cell imaging of Cbln1 exocytosis, pharmacological manipulation (tetanus neurotoxin, lysosome disruptors), Neu1 overexpression, in vivo bouton analysis","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (live imaging, pharmacology, genetic overexpression, in vivo analysis), mechanistic pathway dissected with rigorous controls","pmids":["31072786"],"is_preprint":false},{"year":2017,"finding":"UBE3A (a ubiquitin ligase with transcriptional co-regulatory functions), when overexpressed in the nucleus in VTA glutamatergic neurons, downregulates Cbln1 expression. Cbln1 deletion in VTA glutamatergic neurons impairs sociability and weakens glutamatergic transmission. Viral vector-based restoration of Cbln1 in VTA glutamatergic neurons reverses sociability deficits induced by Ube3a overexpression and/or seizures.","method":"In vivo mouse genetics, conditional gene deletion/overexpression, viral vector-based Cbln1 restoration, chemogenetic activation, behavioral testing, electrophysiology","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal approaches (genetic, viral, chemogenetic, electrophysiology, behavior), in vivo rescue experiment, published in high-impact journal","pmids":["28297715"],"is_preprint":false},{"year":2010,"finding":"The flap loop (Arg321–Trp339) in the N-terminal domain of GluD2 is the critical region for binding to Cbln1 and induction of presynaptic differentiation. Mutations in flap loop residues, including single amino acid substitutions of Arg321 or Trp323 to alanine, abolish both Cbln1 binding and presynaptic differentiation induction in HEK cells expressing GluD2.","method":"HEK cell expression, mutagenesis of GluD2 NTD, Cbln1 binding assay, presynaptic differentiation assay, homology modeling","journal":"Biochemical and Biophysical Research Communications","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — mutagenesis plus functional assay, mechanistic, single lab, single study","pmids":["20599760"],"is_preprint":false},{"year":2020,"finding":"Cbln1/GluD2-dependent competitive interactions shape Purkinje cell dendritic arbor morphogenesis. Sparse but not global GluD2 knockout causes under-elaboration of Purkinje cell dendrites in the deep molecular layer and overelaboration in the superficial molecular layer, demonstrating that competition between granule cell inputs via Cbln1/GluD2 is required for normal dendritic development. Developmental and genetic epistasis analyses confirm this is a synaptotrophic mechanism.","method":"Sparse and global GluD2 conditional knockout, developmental analysis, GluD2 overexpression, structure-function analysis, genetic epistasis, computational modeling","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal genetic approaches (sparse KO, global KO, OE, epistasis), computational modeling validation, rigorous in vivo analysis","pmids":["33352118"],"is_preprint":false},{"year":2022,"finding":"Cbln1 functions as an axon growth and guidance cue during early neural development (distinct from its later synaptogenic role), acting in an autocrine manner on commissural neuron axons to promote axon growth. Cbln1 also acts as an attractive guidance cue from intermediate target tissues. These functions are mediated by neurexin-2 (Nrxn2) as the Cbln1 receptor for axon growth and guidance. Cbln1 also regulates cerebellar parallel fiber growth and retinal ganglion cell axon guidance.","method":"Mouse and chick embryo experiments, loss-of-function and gain-of-function in developing spinal cord, identification of Nrxn2 as receptor by genetic epistasis and binding assays","journal":"PLoS Biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo genetic manipulation in two species, receptor identification by epistasis and binding, single lab","pmids":["36395107"],"is_preprint":false},{"year":2023,"finding":"UBE3A impairs excitatory VMHvl-to-arcuate synapse formation by decreasing Cbln1 gene expression. This synapse is organized by an NRXN1–CBLN1–GluD1 transsynaptic complex. Targeted deletion of Grid1 (GluD1) in arcuate AgRP neurons impairs VMHvl-to-AgRP/NPY excitatory synapses. Chemogenetic/optogenetic activation of arcuate AgRP/NPY neurons inhibits VMHvl neurons and represses aggression, placing the NRXN1–CBLN1–GluD1 complex in a hypothalamic circuit regulating aggression.","method":"Conditional gene deletion, chemogenetic and optogenetic manipulation, behavioral testing, synapse analysis in mouse brain","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple genetic tools and circuit-level manipulation, preprint (not yet peer-reviewed), single lab","pmids":["36909588"],"is_preprint":true},{"year":2025,"finding":"D-serine inhibits the interaction between Cbln1 and GluD1 in a concentration-dependent manner (IC50 ~300 µM) in an in vitro cell-binding assay. In ex vivo central amygdala slices, recombinant Cbln1 increases excitatory neurotransmission and GluD1 expression; this effect is partially blocked by D-serine pre-treatment. The pro-nociceptive behavioral effect of intra-CeA Cbln1 injection is inhibited by D-serine.","method":"In vitro cell-binding assay, ex vivo CeA slice electrophysiology, in vivo intra-CeA injection with behavioral readout","journal":"Cellular and Molecular Life Sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro binding, ex vivo electrophysiology, and in vivo behavior, single lab, multiple orthogonal methods","pmids":["39890638"],"is_preprint":false},{"year":2023,"finding":"YTHDF3 interacts with BTG2 and is involved in the decay of Cbln1 mRNA in the hippocampus, leading to downregulation of Cbln1 protein expression. This identifies YTHDF3/BTG2 as an m6A-dependent post-transcriptional regulatory mechanism controlling Cbln1 levels.","method":"Co-immunoprecipitation (YTHDF3-BTG2 interaction), mRNA decay assays, hippocampal gene expression analysis in prenatal hypoxia model","journal":"iScience","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP and mRNA decay assay, mechanism only partially characterized, single lab","pmids":["38205248"],"is_preprint":false},{"year":2023,"finding":"Cbln1 expressed by CSN subpopulations is sufficient to direct axon extension toward thoraco-lumbar spinal segments. Gain-of-function misexpression of Cbln1 in CSNBC-lat neurons (which normally project only to bulbar-cervical segments) redirects their axons past normal targets toward thoracic segments, demonstrating that Cbln1 is a molecular determinant of segmentally specific corticospinal axon projection targeting.","method":"In utero electroporation, AAV-mediated postmitotic gene delivery, axon tracing in mouse brain, gain-of-function experiments","journal":"The Journal of Neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — gain-of-function with two delivery methods (electroporation + AAV), axon tracing with clear phenotypic readout, single lab","pmids":["36823038"],"is_preprint":false}],"current_model":"Cbln1 is a secreted glycoprotein of the C1q/TNF superfamily that is released from neuronal axons (via lysosomal exocytosis in an activity- and Ca2+-dependent manner) and functions as the central node of an Nrxn–Cbln1–GluD2(or GluD1) trans-synaptic tripartite complex: it bridges presynaptic neurexins to postsynaptic δ-glutamate receptors to bidirectionally organize both excitatory synapse formation/maintenance and inhibitory synapse suppression, and also acts earlier in development as an autocrine axon growth and guidance cue signaling through Nrxn2; its expression is regulated by neuronal activity (via L-type Ca2+ channels and calcineurin), by UBE3A-mediated transcriptional repression, and by YTHDF3/BTG2-dependent mRNA decay, while its protein complex assembly (homomeric hexamers or heteromers with Cbln3) is governed by N-terminal cysteines and the C-terminal C1q domain, with proteolytic processing determining oligomeric state and biological activity."},"narrative":{"mechanistic_narrative":"CBLN1 is a secreted glycoprotein of the C1q superfamily that serves as the central organizing node of a trans-synaptic bridge: it physically links presynaptic neurexins on granule cell axons to the N-terminal domain of postsynaptic δ2-glutamate receptors (GluD2/GluRδ2) on Purkinje cells, thereby driving cerebellar parallel fiber–Purkinje cell synapse formation and maintenance [PMID:20537373, PMID:20395510, PMID:16234806]. Genetic ablation phenocopies GluRδ2-null mice and abolishes synapse maintenance, climbing fiber innervation, and long-term depression, placing CBLN1 in the GluRδ2 signaling pathway [PMID:16234806], and recombinant CBLN1 acutely and reversibly restores parallel fiber synapses and rescues ataxia in cbln1-null animals [PMID:18524896]. CBLN1 binds GluD2 through the receptor's flap loop (Arg321–Trp339), and divergence in C1q-domain loop CD dictates receptor selectivity, while neurexin engages the N-terminal region of the molecule [PMID:28877468, PMID:20599760]. Engagement of the Nrxn–Cbln1–GluD2 complex bidirectionally organizes connectivity, inducing presynaptic differentiation and bouton maturation through positive feedback while simultaneously suppressing inhibitory interneuron–Purkinje cell synapses via a Src-family-kinase-sensitive mechanism, and shaping Purkinje dendritic morphogenesis through competition between inputs [PMID:20395510, PMID:23141067, PMID:24467251, PMID:33352118]. The protein assembles into homomeric hexamers or Cbln3 heteromers governed by N-terminal cysteines and the C-terminal C1q domain, with proteolytic processing determining oligomeric state—only intact hexameric CBLN1 binds postsynaptic sites [PMID:10964938, PMID:16135095, PMID:19344768]. CBLN1 is released from axonal lysosomes in an activity- and Ca2+-dependent manner, retained on the axon surface by neurexin, and accumulates at boutons via GluD2 [PMID:31072786]. Beyond the cerebellum, CBLN1 acts earlier as an autocrine axon growth and guidance cue through neurexin-2 and as a determinant of corticospinal projection targeting, and it organizes hypothalamic and amygdalar excitatory circuits through NRXN1–CBLN1–GluD1 complexes relevant to sociability, aggression, and nociception; its expression is regulated by neuronal activity through L-type Ca2+ channels and calcineurin and by nuclear UBE3A-mediated repression [PMID:19403810, PMID:28297715, PMID:36395107, PMID:36909588, PMID:39890638, PMID:36823038].","teleology":[{"year":2000,"claim":"Establishing the biochemical assembly principles of CBLN1 was the first question: how this secreted protein oligomerizes and whether it associates with related family members.","evidence":"Yeast two-hybrid with Cbln1 bait plus mammalian co-expression and binding specificity assays","pmids":["10964938"],"confidence":"Medium","gaps":["Functional consequence of hexamer vs heteromer not resolved at this stage","Receptor partners unknown","Single lab, no structural data"]},{"year":2005,"claim":"In vivo loss-of-function defined CBLN1's physiological role, showing it is essential for cerebellar synapse maintenance and LTD and operates in the same pathway as GluRδ2.","evidence":"cbln1 knockout mice with genetic epistasis (phenocopy of GluRδ2-null), electrophysiology, morphology","pmids":["16234806"],"confidence":"High","gaps":["Molecular receptor not yet identified","Mechanism of action (direct binding) not yet shown","Did not establish trans-synaptic bridging"]},{"year":2005,"claim":"Proteolytic processing was shown to control which oligomeric form is released, linking biochemistry to biological activity.","evidence":"Biochemical cleavage mapping of cerebellar lysates, yeast two-hybrid, mammalian expression","pmids":["16135095"],"confidence":"Medium","gaps":["Identity of the protease(s) not determined","In vivo relevance of cleavage forms not established","Single lab"]},{"year":2006,"claim":"The interdependence of CBLN1 and CBLN3 secretion was resolved, defining a reciprocal stability/secretion relationship and a structural ER-retention mechanism.","evidence":"cbln1/cbln3 single and double knockout mice, structural modeling, mutation analysis, secretion assays","pmids":["17030622"],"confidence":"High","gaps":["Mechanism applies to cerebellar context; generality unclear","Functional role of the heteromer in synaptogenesis not isolated"]},{"year":2007,"claim":"Subcellular localization placed CBLN1 in the endolysosomal compartment rather than the conventional ER/Golgi secretory route, foreshadowing an unconventional release mechanism.","evidence":"Immunohistochemistry with organelle markers, cbln1-lacZ and cbln3-null mice, fractionation","pmids":["18001291"],"confidence":"Medium","gaps":["Release mechanism not yet defined","Co-localization-based, single lab"]},{"year":2008,"claim":"Recombinant-protein reconstitution proved CBLN1 is the sufficient extracellular factor for synapse formation, establishing it as a soluble synaptogenic cue.","evidence":"Dissociated cbln1-null Purkinje cultures, acute slices, in vivo injection, electrophysiology, EM, behavior","pmids":["18524896"],"confidence":"High","gaps":["Presynaptic partner not yet identified","Transient rescue mechanism unexplained"]},{"year":2009,"claim":"Ultrastructural and trafficking studies localized intact hexameric CBLN1 to the parallel fiber synaptic cleft and demonstrated anterograde trans-neuronal transport requiring GluRδ2.","evidence":"Immunogold EM, binding assays with domain mutants in mutant cerebellum, transgenic and GluRδ2-null mice, fractionation","pmids":["19250438","19200061","19344768"],"confidence":"Medium","gaps":["Mechanism of GluRδ2-dependent uptake unresolved","Functional role of trans-neuronal transport unclear"]},{"year":2009,"claim":"Activity-dependent regulation of cbln1 transcription was established, linking neuronal activity to synapse number via CBLN1 levels.","evidence":"Granule cell cultures with pharmacology (K+, kainate, L-type Ca2+/calcineurin blockers), qRT-PCR, synapse counting, rescue","pmids":["19403810"],"confidence":"Medium","gaps":["Transcription factor downstream of calcineurin not identified","In vivo activity dependence not directly shown"]},{"year":2010,"claim":"The central mechanistic advance: CBLN1 directly binds the GluD2 N-terminal domain and bridges presynaptic neurexins to postsynaptic GluD2, defining the tripartite trans-synaptic organizer and its bidirectional, necessary-and-sufficient synaptogenic activity.","evidence":"cbln1-null cultures, recombinant rescue, NRXN knockdown, domain competition, bead reconstitution, in vivo synapse induction","pmids":["20537373","20395510"],"confidence":"High","gaps":["Atomic-resolution structure of the complex not yet available","Mechanism of presynaptic differentiation downstream unknown"]},{"year":2010,"claim":"Fine-mapping identified the GluD2 flap loop (Arg321–Trp339) as the CBLN1-binding determinant, linking a specific receptor motif to synaptogenic function.","evidence":"HEK cell GluD2 NTD mutagenesis, Cbln1 binding and presynaptic differentiation assays, homology modeling","pmids":["20599760"],"confidence":"Medium","gaps":["Single lab, reciprocal CBLN1 binding residues not mapped here","No co-crystal structure"]},{"year":2012,"claim":"Live imaging defined the dynamic cell-biological output of the complex, showing presynaptic structural remodeling and a positive-feedback loop driving synapse maturation.","evidence":"Time-lapse organotypic imaging, in vivo ultrastructure, GluD2/Nrx genetic epistasis","pmids":["23141067"],"confidence":"High","gaps":["Signaling components of the feedback loop not identified","Cytoskeletal effectors unknown"]},{"year":2012,"claim":"Cross-family redundancy was tested, showing Cbln2 can substitute for CBLN1 in the cerebellum but not in thalamostriatal circuits, revealing region-specific mechanisms.","evidence":"Binding assays, Cbln2 transgenic rescue of cbln1-null mice, cbln2 knockout, synaptic analysis","pmids":["22117778"],"confidence":"Medium","gaps":["Molecular basis of region-specific non-redundancy unknown","Receptor differences in thalamus undefined"]},{"year":2014,"claim":"The organizer was shown to act bidirectionally on connectivity, with Cbln1–GluD2 signaling actively suppressing inhibitory synapses through a Src-family kinase pathway.","evidence":"Patch-clamp in slices, vGAT immunohistochemistry, recombinant rescue, cbln1/GluD2 double-null epistasis, Src inhibitors","pmids":["24467251"],"confidence":"High","gaps":["Src substrate mediating inhibitory suppression not identified","Mechanism linking GluD2 to tyrosine phosphorylation unclear"]},{"year":2017,"claim":"Structural biology resolved the C1q-domain architecture and the basis of receptor selectivity, showing loop CD governs GluD2 binding and that neurexin engages the N-terminal region.","evidence":"X-ray crystallography of Cbln1/Cbln4 C1q domains, negative-stain EM of hexamer and Cbln4/Nrxn1β complex, binding assays","pmids":["28877468"],"confidence":"High","gaps":["Full tripartite complex structure not solved","Low-resolution EM of hexamer leaves quaternary detail unresolved"]},{"year":2017,"claim":"CBLN1 was placed in a transcriptional regulatory and behavioral circuit, with nuclear UBE3A repressing cbln1 in VTA glutamatergic neurons to control sociability.","evidence":"Conditional deletion/overexpression, viral Cbln1 restoration, chemogenetics, behavior, electrophysiology","pmids":["28297715"],"confidence":"High","gaps":["Direct vs indirect transcriptional mechanism of UBE3A on cbln1 not resolved","Receptor partner in VTA circuit not defined here"]},{"year":2019,"claim":"The unconventional release route was defined: CBLN1 is exocytosed from axonal lysosomes in an activity/Ca2+-dependent manner, then captured by neurexin and concentrated at boutons via GluD2.","evidence":"Live-cell exocytosis imaging, tetanus toxin and lysosome-disruption pharmacology, Neu1 overexpression, in vivo bouton analysis","pmids":["31072786"],"confidence":"High","gaps":["Trigger coupling activity to lysosomal fusion not identified","Role of co-released cathepsin B unclear"]},{"year":2020,"claim":"CBLN1/GluD2 signaling was shown to drive competitive, synaptotrophic shaping of Purkinje dendritic arbors, extending its role from synapse counting to circuit morphogenesis.","evidence":"Sparse vs global GluD2 conditional knockout, overexpression, structure-function, epistasis, computational modeling","pmids":["33352118"],"confidence":"High","gaps":["Molecular signal converting synaptic competition to dendritic growth unknown"]},{"year":2022,"claim":"A developmentally earlier function was uncovered: CBLN1 acts as an autocrine axon growth and attractive guidance cue signaling through neurexin-2, distinct from its synaptogenic role.","evidence":"Mouse and chick loss/gain-of-function in developing spinal cord, Nrxn2 receptor identification by epistasis and binding","pmids":["36395107"],"confidence":"Medium","gaps":["Downstream signaling of Nrxn2 in axon growth undefined","How the same protein switches between guidance and synaptogenic modes unclear"]},{"year":2023,"claim":"CBLN1 was shown to be a determinant of corticospinal axon segmental targeting, broadening its role in projection specificity.","evidence":"In utero electroporation, AAV postmitotic delivery, axon tracing, gain-of-function","pmids":["36823038"],"confidence":"Medium","gaps":["Receptor mediating segmental targeting not identified","Endogenous loss-of-function not tested"]},{"year":2023,"claim":"The trans-synaptic module was extended to hypothalamic circuits, with an NRXN1–CBLN1–GluD1 complex organizing VMHvl-to-arcuate excitatory synapses controlling aggression, downstream of UBE3A repression.","evidence":"Conditional deletion, chemo/optogenetics, behavior, synapse analysis (preprint)","pmids":["36909588"],"confidence":"Medium","gaps":["Preprint, not peer-reviewed","Direct CBLN1–GluD1 binding in this circuit not biochemically isolated here"]},{"year":2023,"claim":"A post-transcriptional control layer was proposed, implicating YTHDF3/BTG2 in m6A-dependent decay of cbln1 mRNA.","evidence":"Co-IP (YTHDF3-BTG2), mRNA decay assays, hippocampal expression in prenatal hypoxia model","pmids":["38205248"],"confidence":"Low","gaps":["Single Co-IP without reciprocal validation; direct m6A marking of cbln1 not demonstrated","Functional synaptic consequence not shown","Single lab"]},{"year":2025,"claim":"A small-molecule modulator of the complex was identified, with D-serine inhibiting CBLN1–GluD1 interaction and dampening CBLN1-driven excitatory transmission and nociception in the central amygdala.","evidence":"In vitro cell-binding assay, ex vivo CeA slice electrophysiology, in vivo intra-CeA injection with behavioral readout","pmids":["39890638"],"confidence":"Medium","gaps":["Binding site of D-serine on GluD1 not mapped","Physiological D-serine concentrations relative to IC50 unclear","Single lab"]},{"year":null,"claim":"How CBLN1's distinct functional modes—autocrine axon guidance, region-specific synaptogenesis, and bidirectional excitatory/inhibitory organization—are selected by receptor context, oligomeric state, and proteolytic processing remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No atomic structure of the full Nrxn–Cbln1–GluD complex","Proteases controlling oligomeric state in vivo unidentified","Signal transduction downstream of GluD receptors after CBLN1 binding largely uncharacterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[0,1,11]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[0,1,19,20]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,1,14]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[3,4]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[2,6,8]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[7,15]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[7,9]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[0,1,2,11,13]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[18,19,23]},{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[0,1,11]}],"complexes":["Nrxn–Cbln1–GluD2 trans-synaptic complex","NRXN1–CBLN1–GluD1 trans-synaptic complex","Cbln1 homohexamer","Cbln1–Cbln3 heteromer"],"partners":["GRID2","GRID1","NRXN1","NRXN2","CBLN3","CBLN2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P23435","full_name":"Cerebellin-1","aliases":["Precerebellin"],"length_aa":193,"mass_kda":21.1,"function":"Required for synapse integrity and synaptic plasticity. During cerebellar synapse formation, essential for the matching and maintenance of pre- and post-synaptic elements at parallel fiber-Purkinje cell synapses, the establishment of the proper pattern of climbing fiber-Purkinje cell innervation, and induction of long-term depression at parallel fiber-Purkinje cell synapses. Plays a role as a synaptic organizer that acts bidirectionally on both pre- and post-synaptic components. On the one hand induces accumulation of synaptic vesicles in the pre-synaptic part by binding with NRXN1 and in other hand induces clustering of GRID2 and its associated proteins at the post-synaptic site through association of GRID2. NRXN1-CBLN1-GRID2 complex directly induces parallel fiber protrusions that encapsulate spines of Purkinje cells leading to accumulation of GRID2 and synaptic vesicles. Required for CBLN3 export from the endoplasmic reticulum and secretion (By similarity). NRXN1-CBLN1-GRID2 complex mediates the D-Serine-dependent long term depression signals and AMPA receptor endocytosis (PubMed:27418511). Essential for long-term maintenance but not establishment of excitatory synapses (By similarity). Inhibits the formation and function of inhibitory GABAergic synapses in cerebellar Purkinje cells (By similarity) The cerebellin peptide exerts neuromodulatory functions. Directly stimulates norepinephrine release via the adenylate cyclase/PKA-dependent signaling pathway; and indirectly enhances adrenocortical secretion in vivo, through a paracrine mechanism involving medullary catecholamine release (By similarity)","subcellular_location":"Secreted; Postsynaptic cell membrane","url":"https://www.uniprot.org/uniprotkb/P23435/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CBLN1","classification":"Not Classified","n_dependent_lines":22,"n_total_lines":1208,"dependency_fraction":0.018211920529801324},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CBLN1","total_profiled":1310},"omim":[{"mim_id":"621480","title":"OTOLIN 1; OTOL1","url":"https://www.omim.org/entry/621480"},{"mim_id":"615029","title":"PRECEREBELLIN 4; CBLN4","url":"https://www.omim.org/entry/615029"},{"mim_id":"612978","title":"PRECEREBELLIN 3; CBLN3","url":"https://www.omim.org/entry/612978"},{"mim_id":"602368","title":"GLUTAMATE RECEPTOR, IONOTROPIC, DELTA 2; GRID2","url":"https://www.omim.org/entry/602368"},{"mim_id":"601623","title":"UBIQUITIN-PROTEIN LIGASE E3A; UBE3A","url":"https://www.omim.org/entry/601623"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Cytosol","reliability":"Approved"},{"location":"Flagellar centriole","reliability":"Approved"},{"location":"Mid piece","reliability":"Approved"},{"location":"Principal piece","reliability":"Approved"},{"location":"Perinuclear theca","reliability":"Additional"},{"location":"Calyx","reliability":"Additional"}],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"brain","ntpm":425.4}],"url":"https://www.proteinatlas.org/search/CBLN1"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"P23435","domains":[{"cath_id":"2.60.120.40","chopping":"62-191","consensus_level":"medium","plddt":92.4276,"start":62,"end":191}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P23435","model_url":"https://alphafold.ebi.ac.uk/files/AF-P23435-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P23435-F1-predicted_aligned_error_v6.png","plddt_mean":80.19},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CBLN1","jax_strain_url":"https://www.jax.org/strain/search?query=CBLN1"},"sequence":{"accession":"P23435","fasta_url":"https://rest.uniprot.org/uniprotkb/P23435.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P23435/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P23435"}},"corpus_meta":[{"pmid":"20537373","id":"PMC_20537373","title":"Trans-synaptic 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Cbln1.","date":"2017","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/28297715","citation_count":121,"is_preprint":false},{"pmid":"18524896","id":"PMC_18524896","title":"Cbln1 regulates rapid formation and maintenance of excitatory synapses in mature cerebellar Purkinje cells in vitro and in vivo.","date":"2008","source":"The Journal of neuroscience : the official journal of the Society for Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/18524896","citation_count":87,"is_preprint":false},{"pmid":"10964938","id":"PMC_10964938","title":"Cbln3, a novel member of the precerebellin family that binds specifically to Cbln1.","date":"2000","source":"The Journal of neuroscience : the official journal of the Society for Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/10964938","citation_count":68,"is_preprint":false},{"pmid":"16135095","id":"PMC_16135095","title":"The structure and proteolytic processing of Cbln1 complexes.","date":"2005","source":"Journal of 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Cbln1 induces dynamic axonal structural changes by interacting with GluD2 during cerebellar synapse formation.","date":"2012","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/23141067","citation_count":48,"is_preprint":false},{"pmid":"31072786","id":"PMC_31072786","title":"Activity-Dependent Secretion of Synaptic Organizer Cbln1 from Lysosomes in Granule Cell Axons.","date":"2019","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/31072786","citation_count":44,"is_preprint":false},{"pmid":"33352118","id":"PMC_33352118","title":"GluD2- and Cbln1-mediated competitive interactions shape the dendritic arbors of cerebellar Purkinje cells.","date":"2020","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/33352118","citation_count":38,"is_preprint":false},{"pmid":"19124061","id":"PMC_19124061","title":"New (but old) molecules regulating synapse integrity and plasticity: Cbln1 and the delta2 glutamate 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endolysosomal compartment of neurons.","date":"2007","source":"The European journal of neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/18001291","citation_count":31,"is_preprint":false},{"pmid":"22117778","id":"PMC_22117778","title":"Comparison of Cbln1 and Cbln2 functions using transgenic and knockout mice.","date":"2012","source":"Journal of neurochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/22117778","citation_count":30,"is_preprint":false},{"pmid":"24298240","id":"PMC_24298240","title":"Reevaluation of the role of parallel fiber synapses in delay eyeblink conditioning in mice using Cbln1 as a tool.","date":"2013","source":"Frontiers in neural circuits","url":"https://pubmed.ncbi.nlm.nih.gov/24298240","citation_count":21,"is_preprint":false},{"pmid":"28877468","id":"PMC_28877468","title":"Cbln1 and Cbln4 Are Structurally Similar but Differ in GluD2 Binding Interactions.","date":"2017","source":"Cell 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and guidance in multiple neural regions.","date":"2022","source":"PLoS biology","url":"https://pubmed.ncbi.nlm.nih.gov/36395107","citation_count":14,"is_preprint":false},{"pmid":"24467251","id":"PMC_24467251","title":"Cbln1 downregulates the formation and function of inhibitory synapses in mouse cerebellar Purkinje cells.","date":"2014","source":"The European journal of neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/24467251","citation_count":13,"is_preprint":false},{"pmid":"19344768","id":"PMC_19344768","title":"Characterization of trans-neuronal trafficking of Cbln1.","date":"2009","source":"Molecular and cellular neurosciences","url":"https://pubmed.ncbi.nlm.nih.gov/19344768","citation_count":12,"is_preprint":false},{"pmid":"36823038","id":"PMC_36823038","title":"Cbln1 Directs Axon Targeting by Corticospinal Neurons Specifically toward Thoraco-Lumbar Spinal Cord.","date":"2023","source":"The Journal of neuroscience : the official journal of the Society for Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/36823038","citation_count":11,"is_preprint":false},{"pmid":"19200061","id":"PMC_19200061","title":"Cbln1 binds to specific postsynaptic sites at parallel fiber-Purkinje cell synapses in the cerebellum.","date":"2009","source":"The European journal of neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/19200061","citation_count":11,"is_preprint":false},{"pmid":"29670152","id":"PMC_29670152","title":"Improvement of cerebellar ataxic gait by injecting Cbln1 into the cerebellum of cbln1-null mice.","date":"2018","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/29670152","citation_count":9,"is_preprint":false},{"pmid":"39890638","id":"PMC_39890638","title":"D-Serine disrupts Cbln1 and GluD1 interaction and affects Cbln1-dependent synaptic effects and nocifensive responses in the central amygdala.","date":"2025","source":"Cellular and molecular life sciences : CMLS","url":"https://pubmed.ncbi.nlm.nih.gov/39890638","citation_count":7,"is_preprint":false},{"pmid":"20599760","id":"PMC_20599760","title":"Flap loop of GluD2 binds to Cbln1 and induces presynaptic differentiation.","date":"2010","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/20599760","citation_count":7,"is_preprint":false},{"pmid":"29888788","id":"PMC_29888788","title":"Evidence for generative homology of cerebellum and cerebellum-like structures in an elasmobranch fish based on Pax6, Cbln1 and Grid2 expression.","date":"2018","source":"The Journal of comparative neurology","url":"https://pubmed.ncbi.nlm.nih.gov/29888788","citation_count":7,"is_preprint":false},{"pmid":"38205248","id":"PMC_38205248","title":"YTHDF3 modulates the Cbln1 level by recruiting BTG2 and is implicated in the impaired cognition of prenatal hypoxia 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Population.","date":"2021","source":"Journal of autism and developmental disorders","url":"https://pubmed.ncbi.nlm.nih.gov/34562210","citation_count":3,"is_preprint":false},{"pmid":"36909588","id":"PMC_36909588","title":"UBE3A and transsynaptic complex NRXN1-CBLN1-GluD1 in a hypothalamic VMHvl-arcuate feedback circuit regulates aggression.","date":"2023","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/36909588","citation_count":2,"is_preprint":false},{"pmid":"41144370","id":"PMC_41144370","title":"CBLN1 inhibits the inflammatory response by targeting GluD1 thereby alleviating resiniferatoxin-induced postherpetic neuralgia in mice.","date":"2026","source":"Journal of neuropathology and experimental neurology","url":"https://pubmed.ncbi.nlm.nih.gov/41144370","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.07.16.25331639","title":"Genome-wide study of somatic symptom and related disorders identifies novel genomic loci and map genetic architecture","date":"2025-07-17","source":"bioRxiv","url":"https://doi.org/10.1101/2025.07.16.25331639","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":21130,"output_tokens":7020,"usd":0.084345,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":16171,"output_tokens":6542,"usd":0.122203,"stage2_stop_reason":"end_turn"},"total_usd":0.206548,"stage1_batch_id":"msgbatch_01ACAxJQabDsht39JXeWG22d","stage2_batch_id":"msgbatch_01QEU22A9U5sbPTgsMzTmvSf","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2010,\n      \"finding\": \"Cbln1 forms a trans-synaptic tripartite complex bridging presynaptic neurexins (NRXNs) on granule cells to the N-terminal domain (NTD) of postsynaptic GluRδ2 (GluD2) on Purkinje cells, mediating cerebellar synapse formation. Synaptogenic activity of GluD2 is abolished in cbln1-null cultures and restored by recombinant Cbln1; knockdown of NRXNs in granule cells also abolishes the synaptogenic activity. Soluble NTD of GluD2 and extracellular domain of NRXN1β each suppress Cbln1 synaptogenic activity in vitro and in vivo.\",\n      \"method\": \"Cerebellar primary cultures from cbln1-null mice, recombinant Cbln1 rescue, NRXN knockdown, competitive inhibition with soluble domain fragments, in vivo injection\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (genetic KO, recombinant rescue, domain competition, in vivo), independently replicated across labs\",\n      \"pmids\": [\"20537373\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Cbln1 directly binds the N-terminal domain of GluD2 (the orphan glutamate receptor). The Cbln1–GluD2 complex acts as a bidirectional synapse organizer: Cbln1-coated beads directly induce presynaptic differentiation and indirectly cause postsynaptic molecule clustering via GluD2. GluD2 postsynaptic expression combined with exogenous Cbln1 is necessary and sufficient to induce new synapses in vitro and in adult cerebellum in vivo.\",\n      \"method\": \"Direct binding assays, bead-coated recombinant Cbln1 synaptogenesis assay, in vitro and in vivo synapse induction experiments\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — direct binding shown, bead reconstitution, in vitro and in vivo synaptogenesis, replicated by multiple labs\",\n      \"pmids\": [\"20395510\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Cbln1 is a glycoprotein secreted from cerebellar granule cells that is essential for (1) maintaining pre- and postsynaptic elements at parallel fiber–Purkinje cell synapses, (2) establishing proper climbing fiber–Purkinje cell innervation, and (3) induction of long-term depression at parallel fiber–Purkinje cell synapses. cbln1-null mice phenocopy GluRδ2-null mice, establishing Cbln1 in the same signaling pathway as GluRδ2.\",\n      \"method\": \"cbln1 knockout mice, genetic epistasis (phenocopy of GluRδ2-null), electrophysiology (LTD), morphological analysis\",\n      \"journal\": \"Nature Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean KO with multiple defined phenotypic readouts, genetic epistasis, replicated across multiple subsequent studies\",\n      \"pmids\": [\"16234806\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Cbln1 forms homomeric hexameric complexes and also interacts specifically with Cbln3 to form heteromeric complexes, mediated by conserved N-terminal cysteine residues (for higher-order oligomerization) and the C-terminal C1q domain (for trimerization). C1qB binds to neither Cbln1 nor Cbln3, demonstrating specificity. Cbln3 cannot form stable homomers.\",\n      \"method\": \"Yeast two-hybrid screen using Cbln1 as bait, mammalian co-expression, binding specificity assays\",\n      \"journal\": \"The Journal of Neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — yeast two-hybrid plus mammalian expression, two orthogonal methods, single lab\",\n      \"pmids\": [\"10964938\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Cbln1 undergoes proteolytic processing in the synaptic compartment. Only uncleaved Cbln1 (containing the cerebellin motif) is released and assembles into hexameric complexes. Cleavage at the N-terminus of the cerebellin sequence yields trimeric complexes by separating the C-terminal C1q domain from N-terminal cysteine residues. Cleavage at the C-terminus of the cerebellin motif disrupts the C1q domain and abolishes subunit interactions.\",\n      \"method\": \"Biochemical analysis of cerebellar lysates, yeast two-hybrid, mammalian expression systems, protease cleavage mapping\",\n      \"journal\": \"Journal of Neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro cleavage assays plus yeast two-hybrid and mammalian expression, single lab, two orthogonal methods\",\n      \"pmids\": [\"16135095\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Cbln1 is secreted from cerebellar granule cells in complex with Cbln3. Cbln1 and Cbln3 reciprocally regulate each other's degradation and secretion: cbln1-null mice lack both Cbln1 and Cbln3, whereas cbln3-null mice lack Cbln3 but have ~6-fold increased Cbln1. Cbln3 cannot form homomers and is secreted only when bound to Cbln1. A single arginine residue in Cbln3 causes ER retention via steric clash that is masked upon Cbln1 binding ('hide-and-run' mechanism).\",\n      \"method\": \"cbln1 and cbln3 knockout mice and double knockouts, structural modeling, mutation analysis, biochemical secretion assays\",\n      \"journal\": \"Molecular and Cellular Biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — genetic KOs with defined phenotypes, structure-based mutation analysis, biochemical secretion assays, multiple orthogonal methods\",\n      \"pmids\": [\"17030622\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Recombinant Cbln1 specifically and reversibly induces parallel fiber (PF) synapse formation in dissociated cbln1-null Purkinje cells in culture and rapidly induces functional, ultrastructurally normal PF synapses in acute cbln1-null cerebellar slices. A single injection of recombinant Cbln1 in adult cbln1-null mice in vivo completely but transiently restores PF synapses and rescues ataxia.\",\n      \"method\": \"Dissociated cbln1-null Purkinje cell cultures, acute slice preparations, in vivo injection of recombinant Cbln1, electrophysiology, electron microscopy\",\n      \"journal\": \"The Journal of Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — in vitro reconstitution, ex vivo, and in vivo rescue, multiple orthogonal assays (electrophysiology + ultrastructure + behavior)\",\n      \"pmids\": [\"18524896\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Cbln1 is localized to the endolysosomal compartment of neurons (co-localizing with cathepsin D, a lysosomal marker) but not to ER or Golgi. In cbln3-null cerebellum, Cbln1 immunoreactivity increases dramatically, while it is unchanged in extracerebellar neurons, indicating that Cbln3 regulates Cbln1 levels specifically in the cerebellum via the endolysosomal pathway.\",\n      \"method\": \"Immunohistochemistry with organelle markers, cbln1-lacZ transgenic mice, cbln3-null mice, subcellular fractionation\",\n      \"journal\": \"European Journal of Neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — immunohistochemistry co-localization with validated markers, confirmed using transgenic and KO models, single lab\",\n      \"pmids\": [\"18001291\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Cbln1 accumulates specifically in the synaptic cleft of parallel fiber–Purkinje cell synapses (not other PC synapses), co-localizing with Cbln3 and GluRδ2 at these synapses. Intact hexameric Cbln1 specifically binds to postsynaptic sites; neither N-terminal nor C-terminal fragments alone, nor trimeric mutant Cbln1, support specific binding. Cbln1 binding site is located on postsynaptic (Purkinje cell) rather than presynaptic elements.\",\n      \"method\": \"Postembedding immunogold electron microscopy, pepsin antigen retrieval, binding assays with recombinant Cbln1 in weaver and pcd mutant cerebellum, subcellular fractionation\",\n      \"journal\": \"European Journal of Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (EM immunogold, binding assays with domain mutants, mutant mouse cerebellum, subcellular fractionation), rigorous controls\",\n      \"pmids\": [\"19250438\", \"19200061\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Cbln1 undergoes anterograde trans-neuronal transport from granule cells to Purkinje cells and Bergmann glia, entering the endolysosomal trafficking system. Cbln1 is absent in Purkinje cells and Bergmann glia of GluRδ2-null mice, suggesting GluRδ2 is required for Cbln1 trafficking into postsynaptic cells. Ectopic Cbln1 expression in Purkinje cells (L7-cbln1 transgene) partially rescues locomotor deficits of cbln1-null mice.\",\n      \"method\": \"Immunohistochemistry in transgenic and GluRδ2-null mice, L7-cbln1 transgenic mice, behavioral analysis\",\n      \"journal\": \"Molecular and Cellular Neurosciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — genetic models with defined cellular localization outcomes, single lab, multiple genetic backgrounds tested\",\n      \"pmids\": [\"19344768\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Neuronal activity (elevated K+ or kainate) decreases cbln1 mRNA expression in mature granule cells within hours, in a manner dependent on L-type voltage-dependent Ca2+ channels and calcineurin. Chronic activity also reduces Cbln1 protein levels, accompanied by reduction of excitatory synapses on Purkinje cell dendrites; this activity-induced synapse reduction is prevented by exogenous Cbln1.\",\n      \"method\": \"Granule cell cultures with pharmacological manipulation (K+, kainate, L-type Ca2+ channel blockers, calcineurin inhibitors), qRT-PCR, immunohistochemistry, synapse counting\",\n      \"journal\": \"The Journal of Neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacological dissection of signaling pathway, rescue experiment with recombinant Cbln1, two orthogonal methods, single lab\",\n      \"pmids\": [\"19403810\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Cbln1 released from parallel fibers induces dynamic structural changes (protrusions forming circular structures that encapsulate Purkinje cell spines) in presynaptic parallel fibers through a mechanism requiring postsynaptic GluD2 and presynaptic neurexin (Nrx). Nrx–Cbln1–GluD2 signaling induces accumulation of synaptic vesicles and GluD2, leading to mature synapse formation via a positive feedback mechanism.\",\n      \"method\": \"Time-lapse imaging in organotypic culture, ultrastructural analysis in vivo, genetic manipulation (GluD2/Nrx ablation)\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — live imaging plus ultrastructural analysis plus genetic epistasis (multiple orthogonal methods), single lab\",\n      \"pmids\": [\"23141067\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Cbln1 and Cbln2 have similar binding activities to β-neurexins and Grid2 (GluD2). Ectopic Cbln2 expression in Purkinje cells of transgenic mice rescues cerebellar deficits in cbln1-null mice, demonstrating functional redundancy mediated by common receptor binding properties. However, Cbln2 does not substitute for Cbln1 in thalamic neurons affecting striatal synapses, implying region-specific receptor/mechanism differences.\",\n      \"method\": \"Binding assays, Cbln2 transgenic rescue of cbln1-null mice, cbln2 knockout mice, synaptic analysis\",\n      \"journal\": \"Journal of Neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — transgenic rescue and binding assays, two orthogonal approaches, single lab\",\n      \"pmids\": [\"22117778\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Cbln1–GluD2 signaling downregulates the formation and function of inhibitory synapses between Purkinje cells and interneurons. cbln1-null cerebellum shows increased density of interneuron–Purkinje cell inhibitory synapses and increased amplitude and frequency of miniature IPSCs. Recombinant Cbln1 reverses increased inhibitory currents and synapse density; this effect is absent in cbln1/GluD2 double-null mice. Tyrosine phosphorylation is upregulated in cbln1-null cerebellum, and Src-family kinase inhibition suppresses increased IPSCs.\",\n      \"method\": \"Immunohistochemistry (vGAT antibody), whole-cell patch-clamp in cerebellar slices, recombinant Cbln1 rescue, double-knockout epistasis, Src kinase inhibitors\",\n      \"journal\": \"European Journal of Neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (electrophysiology, morphology, pharmacology, genetic epistasis), rigorous controls including double KO\",\n      \"pmids\": [\"24467251\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Crystal structures of the homotrimeric C1q domains of Cbln1 and Cbln4 resolved at 2.2 Å and 2.3 Å, respectively, reveal that divergence in loop CD accounts for the difference in GluD2 binding between Cbln1 and Cbln4. Negative-stain EM reconstruction of hexameric full-length Cbln1 at 13 Å and Cbln4/Nrxn1β complex at 19 Å shows that Nrxn1β binds to the N-terminal region of Cbln4 (through strand β10 of S4 insert). Cbln4 binds Nrxn1β and forms a stable complex with the LNS domain of Nrxn1β despite not binding GluD2.\",\n      \"method\": \"X-ray crystallography, negative-stain electron microscopy, binding assays\",\n      \"journal\": \"Cell Reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structures plus EM reconstruction plus binding assays, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"28877468\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Cbln1 is released from lysosomes in axons (but not dendrites) of cerebellar granule cells in an activity- and Ca2+-dependent manner. Released Cbln1 is retained on axonal surfaces by binding to presynaptic neurexin, then diffuses laterally and accumulates at boutons by binding postsynaptic δ2 glutamate receptors. Cbln1 exocytosis is insensitive to tetanus neurotoxin, is accompanied by cathepsin B release, and is decreased by lysosome disruption. Overexpression of lysosomal sialidase Neu1 inhibits Cbln1 and cathepsin B exocytosis and reduces axonal bouton formation in vivo.\",\n      \"method\": \"Live-cell imaging of Cbln1 exocytosis, pharmacological manipulation (tetanus neurotoxin, lysosome disruptors), Neu1 overexpression, in vivo bouton analysis\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (live imaging, pharmacology, genetic overexpression, in vivo analysis), mechanistic pathway dissected with rigorous controls\",\n      \"pmids\": [\"31072786\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"UBE3A (a ubiquitin ligase with transcriptional co-regulatory functions), when overexpressed in the nucleus in VTA glutamatergic neurons, downregulates Cbln1 expression. Cbln1 deletion in VTA glutamatergic neurons impairs sociability and weakens glutamatergic transmission. Viral vector-based restoration of Cbln1 in VTA glutamatergic neurons reverses sociability deficits induced by Ube3a overexpression and/or seizures.\",\n      \"method\": \"In vivo mouse genetics, conditional gene deletion/overexpression, viral vector-based Cbln1 restoration, chemogenetic activation, behavioral testing, electrophysiology\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal approaches (genetic, viral, chemogenetic, electrophysiology, behavior), in vivo rescue experiment, published in high-impact journal\",\n      \"pmids\": [\"28297715\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The flap loop (Arg321–Trp339) in the N-terminal domain of GluD2 is the critical region for binding to Cbln1 and induction of presynaptic differentiation. Mutations in flap loop residues, including single amino acid substitutions of Arg321 or Trp323 to alanine, abolish both Cbln1 binding and presynaptic differentiation induction in HEK cells expressing GluD2.\",\n      \"method\": \"HEK cell expression, mutagenesis of GluD2 NTD, Cbln1 binding assay, presynaptic differentiation assay, homology modeling\",\n      \"journal\": \"Biochemical and Biophysical Research Communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — mutagenesis plus functional assay, mechanistic, single lab, single study\",\n      \"pmids\": [\"20599760\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cbln1/GluD2-dependent competitive interactions shape Purkinje cell dendritic arbor morphogenesis. Sparse but not global GluD2 knockout causes under-elaboration of Purkinje cell dendrites in the deep molecular layer and overelaboration in the superficial molecular layer, demonstrating that competition between granule cell inputs via Cbln1/GluD2 is required for normal dendritic development. Developmental and genetic epistasis analyses confirm this is a synaptotrophic mechanism.\",\n      \"method\": \"Sparse and global GluD2 conditional knockout, developmental analysis, GluD2 overexpression, structure-function analysis, genetic epistasis, computational modeling\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal genetic approaches (sparse KO, global KO, OE, epistasis), computational modeling validation, rigorous in vivo analysis\",\n      \"pmids\": [\"33352118\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Cbln1 functions as an axon growth and guidance cue during early neural development (distinct from its later synaptogenic role), acting in an autocrine manner on commissural neuron axons to promote axon growth. Cbln1 also acts as an attractive guidance cue from intermediate target tissues. These functions are mediated by neurexin-2 (Nrxn2) as the Cbln1 receptor for axon growth and guidance. Cbln1 also regulates cerebellar parallel fiber growth and retinal ganglion cell axon guidance.\",\n      \"method\": \"Mouse and chick embryo experiments, loss-of-function and gain-of-function in developing spinal cord, identification of Nrxn2 as receptor by genetic epistasis and binding assays\",\n      \"journal\": \"PLoS Biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo genetic manipulation in two species, receptor identification by epistasis and binding, single lab\",\n      \"pmids\": [\"36395107\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"UBE3A impairs excitatory VMHvl-to-arcuate synapse formation by decreasing Cbln1 gene expression. This synapse is organized by an NRXN1–CBLN1–GluD1 transsynaptic complex. Targeted deletion of Grid1 (GluD1) in arcuate AgRP neurons impairs VMHvl-to-AgRP/NPY excitatory synapses. Chemogenetic/optogenetic activation of arcuate AgRP/NPY neurons inhibits VMHvl neurons and represses aggression, placing the NRXN1–CBLN1–GluD1 complex in a hypothalamic circuit regulating aggression.\",\n      \"method\": \"Conditional gene deletion, chemogenetic and optogenetic manipulation, behavioral testing, synapse analysis in mouse brain\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple genetic tools and circuit-level manipulation, preprint (not yet peer-reviewed), single lab\",\n      \"pmids\": [\"36909588\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"D-serine inhibits the interaction between Cbln1 and GluD1 in a concentration-dependent manner (IC50 ~300 µM) in an in vitro cell-binding assay. In ex vivo central amygdala slices, recombinant Cbln1 increases excitatory neurotransmission and GluD1 expression; this effect is partially blocked by D-serine pre-treatment. The pro-nociceptive behavioral effect of intra-CeA Cbln1 injection is inhibited by D-serine.\",\n      \"method\": \"In vitro cell-binding assay, ex vivo CeA slice electrophysiology, in vivo intra-CeA injection with behavioral readout\",\n      \"journal\": \"Cellular and Molecular Life Sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro binding, ex vivo electrophysiology, and in vivo behavior, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"39890638\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"YTHDF3 interacts with BTG2 and is involved in the decay of Cbln1 mRNA in the hippocampus, leading to downregulation of Cbln1 protein expression. This identifies YTHDF3/BTG2 as an m6A-dependent post-transcriptional regulatory mechanism controlling Cbln1 levels.\",\n      \"method\": \"Co-immunoprecipitation (YTHDF3-BTG2 interaction), mRNA decay assays, hippocampal gene expression analysis in prenatal hypoxia model\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP and mRNA decay assay, mechanism only partially characterized, single lab\",\n      \"pmids\": [\"38205248\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cbln1 expressed by CSN subpopulations is sufficient to direct axon extension toward thoraco-lumbar spinal segments. Gain-of-function misexpression of Cbln1 in CSNBC-lat neurons (which normally project only to bulbar-cervical segments) redirects their axons past normal targets toward thoracic segments, demonstrating that Cbln1 is a molecular determinant of segmentally specific corticospinal axon projection targeting.\",\n      \"method\": \"In utero electroporation, AAV-mediated postmitotic gene delivery, axon tracing in mouse brain, gain-of-function experiments\",\n      \"journal\": \"The Journal of Neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — gain-of-function with two delivery methods (electroporation + AAV), axon tracing with clear phenotypic readout, single lab\",\n      \"pmids\": [\"36823038\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Cbln1 is a secreted glycoprotein of the C1q/TNF superfamily that is released from neuronal axons (via lysosomal exocytosis in an activity- and Ca2+-dependent manner) and functions as the central node of an Nrxn–Cbln1–GluD2(or GluD1) trans-synaptic tripartite complex: it bridges presynaptic neurexins to postsynaptic δ-glutamate receptors to bidirectionally organize both excitatory synapse formation/maintenance and inhibitory synapse suppression, and also acts earlier in development as an autocrine axon growth and guidance cue signaling through Nrxn2; its expression is regulated by neuronal activity (via L-type Ca2+ channels and calcineurin), by UBE3A-mediated transcriptional repression, and by YTHDF3/BTG2-dependent mRNA decay, while its protein complex assembly (homomeric hexamers or heteromers with Cbln3) is governed by N-terminal cysteines and the C-terminal C1q domain, with proteolytic processing determining oligomeric state and biological activity.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CBLN1 is a secreted glycoprotein of the C1q superfamily that serves as the central organizing node of a trans-synaptic bridge: it physically links presynaptic neurexins on granule cell axons to the N-terminal domain of postsynaptic δ2-glutamate receptors (GluD2/GluRδ2) on Purkinje cells, thereby driving cerebellar parallel fiber–Purkinje cell synapse formation and maintenance [#0, #1, #2]. Genetic ablation phenocopies GluRδ2-null mice and abolishes synapse maintenance, climbing fiber innervation, and long-term depression, placing CBLN1 in the GluRδ2 signaling pathway [#2], and recombinant CBLN1 acutely and reversibly restores parallel fiber synapses and rescues ataxia in cbln1-null animals [#6]. CBLN1 binds GluD2 through the receptor's flap loop (Arg321–Trp339), and divergence in C1q-domain loop CD dictates receptor selectivity, while neurexin engages the N-terminal region of the molecule [#14, #17]. Engagement of the Nrxn–Cbln1–GluD2 complex bidirectionally organizes connectivity, inducing presynaptic differentiation and bouton maturation through positive feedback while simultaneously suppressing inhibitory interneuron–Purkinje cell synapses via a Src-family-kinase-sensitive mechanism, and shaping Purkinje dendritic morphogenesis through competition between inputs [#1, #11, #13, #18]. The protein assembles into homomeric hexamers or Cbln3 heteromers governed by N-terminal cysteines and the C-terminal C1q domain, with proteolytic processing determining oligomeric state—only intact hexameric CBLN1 binds postsynaptic sites [#3, #4, #9]. CBLN1 is released from axonal lysosomes in an activity- and Ca2+-dependent manner, retained on the axon surface by neurexin, and accumulates at boutons via GluD2 [#15]. Beyond the cerebellum, CBLN1 acts earlier as an autocrine axon growth and guidance cue through neurexin-2 and as a determinant of corticospinal projection targeting, and it organizes hypothalamic and amygdalar excitatory circuits through NRXN1–CBLN1–GluD1 complexes relevant to sociability, aggression, and nociception; its expression is regulated by neuronal activity through L-type Ca2+ channels and calcineurin and by nuclear UBE3A-mediated repression [#10, #16, #19, #20, #21, #23].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Establishing the biochemical assembly principles of CBLN1 was the first question: how this secreted protein oligomerizes and whether it associates with related family members.\",\n      \"evidence\": \"Yeast two-hybrid with Cbln1 bait plus mammalian co-expression and binding specificity assays\",\n      \"pmids\": [\"10964938\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of hexamer vs heteromer not resolved at this stage\", \"Receptor partners unknown\", \"Single lab, no structural data\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"In vivo loss-of-function defined CBLN1's physiological role, showing it is essential for cerebellar synapse maintenance and LTD and operates in the same pathway as GluRδ2.\",\n      \"evidence\": \"cbln1 knockout mice with genetic epistasis (phenocopy of GluRδ2-null), electrophysiology, morphology\",\n      \"pmids\": [\"16234806\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular receptor not yet identified\", \"Mechanism of action (direct binding) not yet shown\", \"Did not establish trans-synaptic bridging\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Proteolytic processing was shown to control which oligomeric form is released, linking biochemistry to biological activity.\",\n      \"evidence\": \"Biochemical cleavage mapping of cerebellar lysates, yeast two-hybrid, mammalian expression\",\n      \"pmids\": [\"16135095\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of the protease(s) not determined\", \"In vivo relevance of cleavage forms not established\", \"Single lab\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"The interdependence of CBLN1 and CBLN3 secretion was resolved, defining a reciprocal stability/secretion relationship and a structural ER-retention mechanism.\",\n      \"evidence\": \"cbln1/cbln3 single and double knockout mice, structural modeling, mutation analysis, secretion assays\",\n      \"pmids\": [\"17030622\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism applies to cerebellar context; generality unclear\", \"Functional role of the heteromer in synaptogenesis not isolated\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Subcellular localization placed CBLN1 in the endolysosomal compartment rather than the conventional ER/Golgi secretory route, foreshadowing an unconventional release mechanism.\",\n      \"evidence\": \"Immunohistochemistry with organelle markers, cbln1-lacZ and cbln3-null mice, fractionation\",\n      \"pmids\": [\"18001291\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Release mechanism not yet defined\", \"Co-localization-based, single lab\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Recombinant-protein reconstitution proved CBLN1 is the sufficient extracellular factor for synapse formation, establishing it as a soluble synaptogenic cue.\",\n      \"evidence\": \"Dissociated cbln1-null Purkinje cultures, acute slices, in vivo injection, electrophysiology, EM, behavior\",\n      \"pmids\": [\"18524896\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Presynaptic partner not yet identified\", \"Transient rescue mechanism unexplained\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Ultrastructural and trafficking studies localized intact hexameric CBLN1 to the parallel fiber synaptic cleft and demonstrated anterograde trans-neuronal transport requiring GluRδ2.\",\n      \"evidence\": \"Immunogold EM, binding assays with domain mutants in mutant cerebellum, transgenic and GluRδ2-null mice, fractionation\",\n      \"pmids\": [\"19250438\", \"19200061\", \"19344768\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of GluRδ2-dependent uptake unresolved\", \"Functional role of trans-neuronal transport unclear\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Activity-dependent regulation of cbln1 transcription was established, linking neuronal activity to synapse number via CBLN1 levels.\",\n      \"evidence\": \"Granule cell cultures with pharmacology (K+, kainate, L-type Ca2+/calcineurin blockers), qRT-PCR, synapse counting, rescue\",\n      \"pmids\": [\"19403810\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Transcription factor downstream of calcineurin not identified\", \"In vivo activity dependence not directly shown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"The central mechanistic advance: CBLN1 directly binds the GluD2 N-terminal domain and bridges presynaptic neurexins to postsynaptic GluD2, defining the tripartite trans-synaptic organizer and its bidirectional, necessary-and-sufficient synaptogenic activity.\",\n      \"evidence\": \"cbln1-null cultures, recombinant rescue, NRXN knockdown, domain competition, bead reconstitution, in vivo synapse induction\",\n      \"pmids\": [\"20537373\", \"20395510\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure of the complex not yet available\", \"Mechanism of presynaptic differentiation downstream unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Fine-mapping identified the GluD2 flap loop (Arg321–Trp339) as the CBLN1-binding determinant, linking a specific receptor motif to synaptogenic function.\",\n      \"evidence\": \"HEK cell GluD2 NTD mutagenesis, Cbln1 binding and presynaptic differentiation assays, homology modeling\",\n      \"pmids\": [\"20599760\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, reciprocal CBLN1 binding residues not mapped here\", \"No co-crystal structure\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Live imaging defined the dynamic cell-biological output of the complex, showing presynaptic structural remodeling and a positive-feedback loop driving synapse maturation.\",\n      \"evidence\": \"Time-lapse organotypic imaging, in vivo ultrastructure, GluD2/Nrx genetic epistasis\",\n      \"pmids\": [\"23141067\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signaling components of the feedback loop not identified\", \"Cytoskeletal effectors unknown\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Cross-family redundancy was tested, showing Cbln2 can substitute for CBLN1 in the cerebellum but not in thalamostriatal circuits, revealing region-specific mechanisms.\",\n      \"evidence\": \"Binding assays, Cbln2 transgenic rescue of cbln1-null mice, cbln2 knockout, synaptic analysis\",\n      \"pmids\": [\"22117778\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis of region-specific non-redundancy unknown\", \"Receptor differences in thalamus undefined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"The organizer was shown to act bidirectionally on connectivity, with Cbln1–GluD2 signaling actively suppressing inhibitory synapses through a Src-family kinase pathway.\",\n      \"evidence\": \"Patch-clamp in slices, vGAT immunohistochemistry, recombinant rescue, cbln1/GluD2 double-null epistasis, Src inhibitors\",\n      \"pmids\": [\"24467251\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Src substrate mediating inhibitory suppression not identified\", \"Mechanism linking GluD2 to tyrosine phosphorylation unclear\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Structural biology resolved the C1q-domain architecture and the basis of receptor selectivity, showing loop CD governs GluD2 binding and that neurexin engages the N-terminal region.\",\n      \"evidence\": \"X-ray crystallography of Cbln1/Cbln4 C1q domains, negative-stain EM of hexamer and Cbln4/Nrxn1β complex, binding assays\",\n      \"pmids\": [\"28877468\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full tripartite complex structure not solved\", \"Low-resolution EM of hexamer leaves quaternary detail unresolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"CBLN1 was placed in a transcriptional regulatory and behavioral circuit, with nuclear UBE3A repressing cbln1 in VTA glutamatergic neurons to control sociability.\",\n      \"evidence\": \"Conditional deletion/overexpression, viral Cbln1 restoration, chemogenetics, behavior, electrophysiology\",\n      \"pmids\": [\"28297715\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct vs indirect transcriptional mechanism of UBE3A on cbln1 not resolved\", \"Receptor partner in VTA circuit not defined here\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"The unconventional release route was defined: CBLN1 is exocytosed from axonal lysosomes in an activity/Ca2+-dependent manner, then captured by neurexin and concentrated at boutons via GluD2.\",\n      \"evidence\": \"Live-cell exocytosis imaging, tetanus toxin and lysosome-disruption pharmacology, Neu1 overexpression, in vivo bouton analysis\",\n      \"pmids\": [\"31072786\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Trigger coupling activity to lysosomal fusion not identified\", \"Role of co-released cathepsin B unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"CBLN1/GluD2 signaling was shown to drive competitive, synaptotrophic shaping of Purkinje dendritic arbors, extending its role from synapse counting to circuit morphogenesis.\",\n      \"evidence\": \"Sparse vs global GluD2 conditional knockout, overexpression, structure-function, epistasis, computational modeling\",\n      \"pmids\": [\"33352118\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular signal converting synaptic competition to dendritic growth unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"A developmentally earlier function was uncovered: CBLN1 acts as an autocrine axon growth and attractive guidance cue signaling through neurexin-2, distinct from its synaptogenic role.\",\n      \"evidence\": \"Mouse and chick loss/gain-of-function in developing spinal cord, Nrxn2 receptor identification by epistasis and binding\",\n      \"pmids\": [\"36395107\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Downstream signaling of Nrxn2 in axon growth undefined\", \"How the same protein switches between guidance and synaptogenic modes unclear\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"CBLN1 was shown to be a determinant of corticospinal axon segmental targeting, broadening its role in projection specificity.\",\n      \"evidence\": \"In utero electroporation, AAV postmitotic delivery, axon tracing, gain-of-function\",\n      \"pmids\": [\"36823038\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor mediating segmental targeting not identified\", \"Endogenous loss-of-function not tested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"The trans-synaptic module was extended to hypothalamic circuits, with an NRXN1–CBLN1–GluD1 complex organizing VMHvl-to-arcuate excitatory synapses controlling aggression, downstream of UBE3A repression.\",\n      \"evidence\": \"Conditional deletion, chemo/optogenetics, behavior, synapse analysis (preprint)\",\n      \"pmids\": [\"36909588\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, not peer-reviewed\", \"Direct CBLN1–GluD1 binding in this circuit not biochemically isolated here\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"A post-transcriptional control layer was proposed, implicating YTHDF3/BTG2 in m6A-dependent decay of cbln1 mRNA.\",\n      \"evidence\": \"Co-IP (YTHDF3-BTG2), mRNA decay assays, hippocampal expression in prenatal hypoxia model\",\n      \"pmids\": [\"38205248\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single Co-IP without reciprocal validation; direct m6A marking of cbln1 not demonstrated\", \"Functional synaptic consequence not shown\", \"Single lab\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"A small-molecule modulator of the complex was identified, with D-serine inhibiting CBLN1–GluD1 interaction and dampening CBLN1-driven excitatory transmission and nociception in the central amygdala.\",\n      \"evidence\": \"In vitro cell-binding assay, ex vivo CeA slice electrophysiology, in vivo intra-CeA injection with behavioral readout\",\n      \"pmids\": [\"39890638\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Binding site of D-serine on GluD1 not mapped\", \"Physiological D-serine concentrations relative to IC50 unclear\", \"Single lab\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CBLN1's distinct functional modes—autocrine axon guidance, region-specific synaptogenesis, and bidirectional excitatory/inhibitory organization—are selected by receptor context, oligomeric state, and proteolytic processing remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No atomic structure of the full Nrxn–Cbln1–GluD complex\", \"Proteases controlling oligomeric state in vivo unidentified\", \"Signal transduction downstream of GluD receptors after CBLN1 binding largely uncharacterized\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [0, 1, 11]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [0, 1, 19, 20]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 1, 14]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [3, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [2, 6, 8]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [7, 15]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [7, 9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [0, 1, 2, 11, 13]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [18, 19, 23]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [0, 1, 11]}\n    ],\n    \"complexes\": [\n      \"Nrxn–Cbln1–GluD2 trans-synaptic complex\",\n      \"NRXN1–CBLN1–GluD1 trans-synaptic complex\",\n      \"Cbln1 homohexamer\",\n      \"Cbln1–Cbln3 heteromer\"\n    ],\n    \"partners\": [\n      \"GRID2\",\n      \"GRID1\",\n      \"NRXN1\",\n      \"NRXN2\",\n      \"CBLN3\",\n      \"CBLN2\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}